1. Field of the Invention
The present invention relates to electric power transmission systems, especially those employing overhead electric power lines, and deals more particularly with a system for determining the present un-stretched length of a conductor in the span between support towers at a reference temperature from a measurement of a present inclination angle and the present conductor temperature. Knowledge of the present un-stretched length of the conductor at a reference temperature enables the computation of the maximum load current that the conductor can presently carry for the existing weather conditions without violating a pre-determined amount of conductor sag or clearance between the conductor and objects that exist below the conductor. The maximum load determination is performed on a real-time basis by taking into consideration at least the thermal effects of line current, weather conditions, solar radiation, and the present state of the conductor which includes the effect of conductor creep that may have occurred since the conductor was initially installed.
The present invention further relates to a specific calculation methodology to arrive at a maximum allowable conductor temperature that corresponds to a clearance limitation, between the conductor and the ground or objects directly beneath the conductor, in real time, even as the conductor is experiencing ongoing creep.
2. Description of the Related Art
As the load on an electric power system grows, the line current increases and energy losses become greater. The line current increases also have an adverse effect on clearance limitations of the power transmission lines. The load is measured in terms of the product of volts (V) and current (I), or VA. In the past it has been standard practice to increase the voltage level of the power transmission lines in order to meet growing consumer and business demands, thereby lowering the current and minimizing the energy losses.
However, this approach may be undesirable because of the potential adverse environmental effects of the higher voltage levels, including high electric fields, radio and television interference, audible noise and induced voltage. If higher voltage levels are not employed to satisfy increased demands, one option to power utility companies is to increase the current of the transmission line. However, increasing the current carried by the transmission line produces higher energy losses, which leads to higher conductor temperatures, increased sag and smaller clearance of the power transmission lines.
To effectively utilize overhead electrical power transmission lines, it is necessary to determine their actual thermal capacity in real time which in turn determines the maximum amount of electrical current that the transmission lines may safely carry to control sag and comply with minimum clearance restrictions. In the past, design ratings for the lines have been derived from theoretical calculations based on assumed weather conditions and selected values of conductor temperature. Safe values of conductor temperature are based on line clearance requirements, loss of tensile strength, and creep criteria.
Weather conditions substantially affect the current carrying capacity of an overhead electrical power line. Theoretical calculations are normally based on assumptions of low wind speeds perpendicular to the conductor, high ambient temperatures and maximum solar radiation. As a result, the calculation for arriving at a design rating is based on the assumption that the weather has a minimum cooling effect on the conductor while maximizing the amount of heat absorbed by the conductor. This ensures that the line temperature is normally less than the highest attainable temperature when the line is carrying the rated load, the sag of the line is prevented from exceeding a pre-selected safe clearance above the ground, and the conductor is prevented from losing more than an acceptable amount of tensile strength.
However, this assumption is not always accurate because it is based on assumed wind speeds and ambient temperatures. Therefore, by employing such approximations, the sag cannot be determined accurately on a real-time basis, which is critical when loading is at or near the rating of the line.
One known system for rating electric power transmission lines and equipment is disclosed in U.S. Pat. No. 4,806,855. The system described in the '855 patent determines the current carrying capability of one or more overhead power transmission lines on a real-time basis by taking into consideration the thermal effects of line current, wind velocity, wind direction, solar radiation and ambient temperature on the line conductor.
Another system for rating electric power transmission lines and equipment is disclosed in U.S. Pat. No. 5,140,257. The '257 patent describes a system where the thermal state of each monitored line span is determined by measuring the conductor temperature, line current, solar radiation, ambient temperature, and in some cases wind speed and wind direction.
These parameters are monitored by a sensor-transmitter unit that may be removably clamped on the line conductor, which may range in size from one half inch to several inches in diameter, and includes a radio transmitter for transmitting sensed data to a receiving substation. The data from the sensor-transmitter is transmitted by a telecommunications link to a computer, which automatically determines line capacity using the real-time data. The computer also calculates the time required for the “critical span” having the lowest current capacity to reach its maximum safe temperature based on any of a number of step changes in load demands.
Still another system is disclosed in U.S. Pat. No. 5,341,088. The system of the '088 patent provides, in one embodiment, an inclinometer that is installed in a line-mounted sensor that senses the angle of inclination of the overhead line conductor at the point where the sensor is installed, and the value of this inclination is used to compute, in real time, the maximum allowable conductor temperature that the line can experience without violating its minimum clearance. The angle of the line is used to compute the sag of the line at the conductor temperature that exists at the time of the measurement, which is then used to determine the maximum allowable conductor temperature.
However, the '855, '257 and '088 patents, do not describe a calculation methodology to arrive at the maximum allowable conductor temperature in real time that corresponds to the clearance limitation and do not take into consideration the ongoing creep.
As is apparent from the foregoing, the factors that limit the current rating to a safe value are a function of conductor temperature. Thus, if the line conductor temperature and weather conditions are closely and accurately monitored on a real-time basis, then the maximum real-time current could be substantially greater than the conservative design rating for a large portion of time during the year.
For the foregoing reasons, there is a need for a system and method for determining the maximum allowable conductor temperature that corresponds to the minimum conductor clearance, in real time, even as the conductor is experiencing ongoing creep.